Here is my understanding:

Recently, a security vulnerability of Lemmy has been exploited by some malicious actors. This lead to some instances going down. The vulnerability has been fixed with version 0.18.2-rc.1 of lemmy-ui. But due to the way Lemmy issues and uses access tokens, the sessions has been invalidated in the database. So, the admins are recommending the users to log out and log back in if they haven’t done so after the upgrade to version 0.18.2-rc.1 of lemmy-ui.

But I may be wrong. Perhaps others can provide a more accurate description.